(a) Technical Field
The present disclosure relates to an air supply apparatus and method for a fuel cell. More particularly, the present disclosure relates to an apparatus and method for a fuel cell, which supplies high-pressure air while avoiding a surge phenomenon of a turbo type compressor in a fuel cell system to which the turbo type compressor is applied.
(b) Background Art
Fuel cell systems generally convert chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. A fuel cell generally consists of an anode, a cathode and an electrolyte that allows charges to move between the two sides of the fuel cell. In particular, electrons are drawn from the anode to the cathode through an external circuit, producing direct current electricity that can be used to power various devices or machinery, such as vehicles.
In order to provide a sufficient amount of electricity, fuel cells in a fuel cell system are often stacked to form a fuel cell stack. Also included in the system is a fuel supply apparatus for supplying fuel to the stack, an air supply apparatus for supplying air to the stack, and an exhaust apparatus for discharging moisture, air and fuel which are generated in the power generation reaction of the stack.
The air supply apparatus is very significant to the overall operation of the fuel cell. This apparatus consumes about 5% to about 20% of the maximum output of the fuel cell system. Also, the operation efficiency of the air supply apparatus is sensitive to the operating pressure due to the characteristics of apparatuses dealing with a compressible fluid.
Various types of air supply apparatuses have been used in the fuel cell systems. Some examples are disclosed in Japanese Patent Application Publication Nos. 2009-123550 and 2008-103228, and Korean Patent Application Publication No. 10-2013-0078697.
In order to efficiently operate a typical fuel cell system while providing an output required for the fuel cell system according to external environmental conditions such as atmospheric pressure and atmospheric humidity according to the altitude of the vehicle and an output desired by a driver, the operation conditions (i.e., the operation temperature, operation pressure, air volume/air Stoichiometric Ratio (SR), and Relative Humidity (RH) of supplied air) need to be optimized.
For example, when a desired output increases under designed optimal operation conditions (i.e., the operation pressure according to the operation temperature, air SR, etc.), the heat radiation load increases, allowing the operation temperature to exceed the standard operation temperature (desired operation temperature or optimal temperature).
In this case, for the maintenance of smooth humidification (maintenance of water balance) and the improvement of the system efficiency, it is effective to increase the operation pressure (air supply pressure) and reduce the air SR (air volume).
FIG. 4 is a schematic view illustrating a typical air supply apparatus for a fuel cell. As shown in FIG. 4, in the air supply apparatus for the fuel cell, a compressor 120 is disposed on an air inflow line 110 of a fuel cell stack 100, and a bypass line 140 connects between the air inflow line 110 and an air outflow line 130 of the fuel cell stack 100. In this case, a to bypass valve 150 is disposed on the bypass line 140. Unexplained reference numerals 160 and 170 denote air flow meters, and reference numerals 180 and 190 denote a pressure sensor and a pressure control valve, respectively.
As shown in FIG. 5, the operation condition desired by the stack is a condition I, and the operable condition of the turbo compressor while satisfying the flow rate of the condition I is a condition J, but the condition J is not satisfied. In particular, when the operation condition is a condition K, the pressure may be met, but excessive air is supplied.
Accordingly, to avoid a surge and to satisfy the operation condition I desired by the stack, the turbo compressor operates under the condition K, and excessive air (e.g., difference of air volumes between points I and K) is discharged through a bypass to supply an air pressure and a flow rate that corresponds to the condition I to the stack. Additionally, when the operation temperature reaches the standard temperature (e.g., desired operation temperature or optimal temperature), and the operation condition necessary for the stack or the fuel cell system falls within the operation region (e.g., normal operation line at the right side of the surge line) of the turbo compressor, the bypass valve is closed.
Accordingly, stable operation may be achieved while avoiding the surge conditions of the air supply apparatus, by opening the bypass valve and thus changing the operation line as shown in FIG. 5. Furthermore, under high-humidity (e.g., designed operation conditions including humidity) or other conditions, the bypass valve remains or is closed. Thus, the power consumption of the compressor is reduced.
As another example, Japanese Patent Application Publication No. 2004-235076 to discloses a device in which a discharge part of a compressor suctioning, compressing, and supplying air to a fuel cell stack stores a portion of the air in an accumulator, and when the atmospheric pressure is reduced, the stored air is discharged into a suction part of the compressor from the accumulator to increase the suction pressure of the compressor.
However, some fuel cell systems use a turbo type compressor to optimize the operation conditions of the fuel cell system according to external environmental conditions and the output desired by a driver. In these types of systems, the fuel cell system may be required to operate at a substantially high pressure and relatively low flow conditions. In this case, a surge phenomenon occurs in the turbo type compressor, causing instability of the whole system. Thus, an air supply system that can be used efficiently and effectively in turbo compressor fuel cell system is required.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.